Electric propulsion systems (EPS) are used to supplement chemical propulsion in orbital and attitude control of satellites. Although generating low thrust, the EPS is efficient and useful for certain applications, such as attitude and stationkeeping control. The task in this particular case is orbit adjustment from insertion to operating orbit through a series of transition orbits. The use of EPS for this purpose will reduce the operation of the chemical thrusters with a savings in mass. This allows the chemical thruster system to be used for other maneuvers or allows a reduction in fuel. The mass savings could provide space for additional payload or could be used to extend the on orbit fuel lifetime of the satellite.
The use of electric propulsion to obtain final orbit is described in some detail in U.S. Pat. No. 5,716,029, which issued to Spitzer, et al. In the system described in Spitzer, et al, a satellite is transferred from its injected orbit into a supersynchronous intermediate orbit by its chemical propulsion system. A Xenon ion propulsion system is used to move the satellite into its operational orbit from the intermediate orbit. The EPS applies a thrust which is constant in both amplitude and direction. In the Spitzer, et al system, therefore, the attitude of the satellite remains fixed during the orbit transition process. This approach tends to sacrifice thrust efficiency for maintenance of maximum solar power.
It is a purpose of this invention to provide a electric propulsion system which is controlled to optimize thrust efficiency.
The EPS provides a low amplitude thrust, but requires a large amount of electrical power. This power is supplied by the onboard solar energy generating system which may typically provides more than 15 kw of power. The EPS may use more than one half of the available solar power. Although the EPS power drain in this particular application will precede the full operation of the satellite payload, maximum solar power generation is desirable during the orbital transfer, especially when considering power for buss electronics, heaters, battery charge power, and similar power drains. This requires that the solar array be oriented as near to perpendicular to the sun at all times as can be achieved.
The use of the EPS for the described purpose sacrifices transfer orbit duration for reduction of mass. An orbit transfer process, that can be accomplished by chemical propulsion in a week's time, becomes a three month journey with EPS. It is therefore critical that efficient use of the EPS be maintained at all times, otherwise the transfer from a transitional orbit to operational orbit could be extended for a considerable period.
The use of a constant magnitude thrust vector in a fixed direction, in accordance with Spitzer, may simplify control, but it requires continuous monitoring by ground control. Ground intervention is required to redirect the thrust vector when efficiency becomes unacceptable, hence the preoccupation of Spitzer with a 24 hour orbit.
It is a purpose of this invention to provide a control system which prioritizes for optimum thrust vector direction at all times while allowing for orientation of the solar array for maximum power generation.